Previous studies with laboratory animals have shown that periods of hours between performance of required avoidance tasks are associated with sustained inhibition of breathing frequency below resting levels, together with a pressor response. This pattern potentiates the development of experimental hypertension in animals when combined with a high sodium intake. This project investigates the relevance of this paradigm to regulation of human blood pressure. An ambulatory respiration monitor has been developed and refined, which has shown that some humans exhibit periods of inhibitory breathing (subnormal frequency without compensation in tidal volume) in the natural environment. These episodes occur more frequently when subjects are at work than when they are at home, and more frequently when they are with others than when they are alone. Inhibitory breathing by humans has also been found to be accompanied by increased blood pressure, but not heart rate. The blood pressure response is not, however, mediated via muscle sympathetic activity, suggesting that it may represent an effect of blood gas changes on vascular tone. Thus, a physiological pattern has been described that is alternative to the "fight or flight" reflex and that, in combination with high sodium intake, may participate in the development of hypertensive adaptations in humans.